Date of Award

2020

Degree Type

Thesis

Degree Name

Master of Science in Biological and Environmental Sciences (MSBES)

Specialization

Ecology & Ecosystem Sciences (EES)

Department

Biological Sciences

First Advisor

Evan L. Preisser

Abstract

Eastern hemlock (Tsuga canadensis), a canopy-dominant conifer native to the eastern U.S., is currently threatened with extirpation by the invasive stylet-feeding hemlock woolly adelgid (Adelges tsugae). To understand interactions between eastern hemlock and hemlock woolly adelgid, and their interactions with other forest defoliators, we carried out two experiments.

First, in 2018, we evaluated the impact of feeding by hemlock woolly adelgid on gypsy moth (Lymantria dispar) larval preference for, and performance on, eastern hemlock. To assess preference, we surveyed 245 field-grown hemlocks for gypsy moth herbivory damage and conducted laboratory paired-choice bioassays. To assess performance, gypsy moth larvae were reared to pupation on adelgid-infested or uninfested hemlock foliage and pupal weight, proportional weight gain, and larval period were analyzed. Adelgid-infested hemlocks experienced more gypsy moth herbivory than uninfested control trees, and laboratory tests confirmed that gypsy moth larvae preferentially feed on adelgid-infested hemlock foliage. Gypsy moth larvae reared to pupation on adelgid-infested foliage gained more weight than larvae reared on uninfested control foliage. Our results suggest that the synergistic effect of adelgid and gypsy moth poses an additional threat to eastern hemlock that may increase extirpation risk and ecological impact throughout most of its range.

Second, we have conducted over a decade of research into rare eastern hemlock (Tsuga canadensis; hemlock) trees that appear resistant to hemlock woolly adelgid (Adelges tsugae; HWA). Following clonal propagation of these rare individuals, in 2015 we planted size- and age-matched HWA-resistant and HWA-susceptible hemlocks in HWA-infested forest plots in seven states. In 2019, we re-surveyed the plots; 96% of HWA-resistant hemlocks survived compared to 48% of susceptible trees. The surviving HWA-resistant trees were also taller, produced more lateral growth, retained more foliage, and supported lower elongate hemlock scale (Fiorinia externa) and HWA densities than the surviving HWA-susceptible trees. Our results suggest that HWA management may benefit from additional research exploring the identification, characterization, and use of HWA-resistant eastern hemlocks in future reforestation efforts.

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